I can understand the advantage of a MicroSD slot, but I've never understood why so many people consider a removable battery such an advantage.
I have never swapped batteries on my GS3 or my Note 2, it just isn't feasible to carry a spare fully charged battery with you at all times to make a removable battery a real advantage.

Anyway, I've tried both phones, and they are both amazing devices, but if I had the choice of only one device, I'd lean towards the HTC One, it's something different and the build quality is impeccable.

There is a game called Ingress. It will chew through your battery. You have two choices.. well three. Buy another battery as a spare, use the stock battery and rely on an external battery pack. Or buy a spare and a external battery pack.